Modular systems consist of a number of modules interconnected via a backplane. For this purpose, the backplane has a number of slots into which the individual modules can be plugged in a modular manner. Generally the backplane provides the voltage supply for these individual modules via these slots. In addition, the slots are interconnected using appropriate electrical or optical media in such a way that the signals transmitted by one of the modules can be received by the other modules and vice versa, the transmitted and received signals containing information such as a code for addressing or also data for data communication between the modules.
EP 0 237 236 discloses the use of a backplane having an optical waveguide for data communication between the modules. For this purpose the optical waveguide has, at defined intervals along its length, appropriate means of coupling out portions of the light guided in the optical waveguide and allowing the rest of the light through. These coupled-out light portions additionally contain the complete information transmitted in the optical signals. The coupled-out light, and therefore the optical signals, are then guided to the individual modules via other optical elements such as lenses. Through this arrangement of the means in the optical waveguide, portions of the light remaining in the optical waveguide are thus repeatedly coupled out. Particularly if there are a large number of slots, the intensity of the light is therefore reduced more and more. This has the disadvantage that the farther modules in the propagation direction of the light receive the information contained in the coupled-out optical signals with ever lower optical intensity. As the number of modules increases, the signal-to-noise ratio continues to deteriorate, possibly resulting in defective information transmission.
WO 88/08573 also describes a modular system with a number of modules which are plugged into a backplane. The inserted modules can communicate with one another accordingly via the backplane. Each module additionally has an optical emitter and an optical receiver which are disposed in such a way that, in the inserted state, a module's optical emitter is always opposite the optical receiver of the immediately adjacent module. To address the modules, the first module then sends a code for a possible valid address via the backplane. The first module simultaneously activates its optical emitter. The optical receiver of the second module is therefore illuminated and activated via the free air interface between the first and the adjacent second module. The activated optical receiver then enables the code transmitted via the backplane for further processing on the second module. When this second module has adopted the code of the valid address, it activates its optical emitter. The latter in turn illuminates the optical receiver of the following third module via another air interface, and so on. This kind of series connection of modules for addressing is known as “daisy chaining”. The arrangement shown in WO 88/08573 has the disadvantage, however, that further optical emitters and receivers in addition to the backplane are necessary for addressing.